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The second (symbol: s), is a unit of time. There are 60 seconds in a minute, 60 minutes in an hour, and 24 hours in a day. This tradition dates back to the Babylonian.
In science, a second is the time it takes for a caesium atom to vibrate 9,192,631,770 (around 9 billion) times. Scientists measure the second this way because the length of a day changes all the time. For example, when the dinosaurs lived, a day was about an hour shorter. Vibrations of atoms on the other hand always take the same time. This atomic second is also called the SI second.
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